I'm by no means an expert in this game, but I found IGN's walkthru to be very detailed and full of good advice for this difficulty level.
Updated link: http://uk.ign.com/wikis/ninja-gaiden-ii/Walkthrough
Obviously it contains spoilers. It also includes a guide for the crystal skulls along the way, but does not include guides for the Tests of Valor (other than where they're located on each level).
This guide will have you using the Dragon Sword and the Eclipse Scythe for the majority of the game and so you won't need to spend a lot of yellow essence on upgrades for other weapons.